1. Field of the Invention
Embodiments of the invention generally relate to electrochromic devices and materials and processes for forming such electrochromic devices and materials.
2. Description of the Related Art
Electrochromic refers to the ability of certain materials to change color when an electrical charge is applied. In the 1930s, bulk tungsten oxide was first shown to change color electrochemically. Electrochromic switchable rearview mirrors based on tungsten oxide and viologens were developed successfully in the 1980s and are used widely in automobiles today. In the 1990s, development of electrochromic thin films for architectural glazing was initiated by several companies, although their efforts have not yet resulted in widespread applications in the building industry primarily because of the high cost and durability issues.
There are different types of electrochromic materials and structures which are used for different application purposes. Inorganic and substantially inorganic electrochromic thin film materials, superimposed in a multilayer structure, are typically used for architectural window applications because of a high durability with respect to heat and ultraviolet radiation. Although electrochromic rearview mirrors for automobiles based on liquid or polymeric electrochromes have been successfully marketed, more widespread use of electrochromic devices in applications such as eyewear, display, architectural windows and skylights, etc. have not been realized, mainly because of the high cost, low cycling durability, and inadequate switching range and speed.
Various different deposition techniques have been used to date to fabricate EC thin films. Among the most popular techniques are vacuum deposition techniques such as sputtering, thermal and electron beam evaporation, and chemical vapor deposition (CVD). These deposition techniques, especially for large-area applications, suffer from at least one of the following problems: i) complex and expensive vacuum system; ii) slow deposition rate; iii) expensive source materials; iv) expensive maintenance; v) nonuniformity of the deposited films; and vi) low yield.
Alternatively, wet deposition techniques have also been used. Sol-gel techniques have been used to deposit some of the layers within a EC device. While sol-gel is a cost-effective technique, it suffers from drawbacks such as slow throughput and significant chemical solution waste. Besides sol-gel, electrodeposition also is used to deposit both the primary and secondary EC layers. Although electrodeposition can be inexpensive, it is difficult to deposit mixtures of metal oxides, and multilayers, as well as to maintain good film uniformity across large-area devices. Besides sol-gel and electrodeposition, spray pyrolysis (SP) has been used to produce single EC layers at different substrate temperatures. The EC layers formed by SP might be exposed to a heat treatment after the SP process—which leads to increased time and cost. To fabricate EC devices, single EC layers are usually laminated together via a polymer ion conductor or an organically modified electrolyte (ormolyte). Lamination increases the production complexity and hence cost, as well as adding additional weight which can be substantial if glass substrates are utilized within the EC devices. Furthermore, organic or ormolyte ion conductors increase the risk of degradation when exposed to heat and/or ultraviolet radiation.
Roughness of the substrate or one or more of the layers in an electrochromic multilayer stack can affect the other layers deposited above, increasing light scattering and haze, as well as increasing pinholes and electrical shorts.
Therefore, there is a need for electrochromic devices and materials with improved properties, such as higher switching range, speed, and cycling durability, as well as minimized light scattering, haze, pinholes, and electrical shorts. Also, a process for forming such electrochromic devices and materials at a reduced cost of manufacturing and increased throughput is desirable over previous processes, especially for large-area applications.